1. Field of the Invention
The present invention relates to a tire for motor vehicles, and more particularly to a tire for heavy duty vehicles such as lorries and the like which are intended for mixed operation, in other words for use both on roads with good surface conditions and or rough and irregular grounds such as those generally present in quarries and/or on building sites.
2. Description of the Related Art.
As is known, heavy duty vehicles such as lorries are generally fitted with tyres of different types according to whether they are to be fitted on the traction (driving) axles or on the driven (steering) axles.
This differentiation is due to the different performances and behavioural characteristics required from tyres according to the axle on which they are fitted and on the type of operation for which they are intended.
For example, for operation predominantly on roads with good surface conditions, the tyres fitted on the driving wheels, generally the rear wheels, are mainly required to have good traction in any road surface condition, whether dry, wet or icy, and a good regularity of wear which provides a high mileage.
For this purpose, tyres with a tread of the so-called “lug” type are used, in other words tyres having a tread provided with a plurality of lugs formed by a combination of circumferential channels intersected by transverse channels, of considerable width, normally not less than approximately 5 mm. The dimensions and orientation of the transverse channels have a considerable effect on the ground-gripping action exerted by the tread.
On the other hand, the tyres fitted on the driven axles, generally the front axles, are primarily required to have a high directional stability, good grip and lateral roadholding, together with low abradability and high tear resistance, to provide a good mileage.
For this purpose, therefore, use is made of tyres having a tread of the so-called “grooved” type, in other words a tread provided with grooves of the circumferential type, with a rectilinear or zigzag configuration, which form on the tread a plurality of circumferential ribs, in other words a plurality of continuous circumferential strips which do not have transverse cuts extending over their whole width.
Conversely, for operation on rough and irregular surfaces, and also on muddy and/or sandy surfaces, tyres of the lug type are predominantly used on the front axles as well, although the transverse cuts delimiting said lugs are normally narrower than the transverse cuts of the tyres for use on the driving axles. Thus, there are known tyres whose tread comprises circumferential grooves with a zigzag configuration, creating a plurality of circumferential lugs delimited circumferentially by sipes, in other words by very narrow transverse cuts, each of which interconnects two adjacent circumferential grooves.
This type of tread is described, for example, in EP-384,182 in relation to a tyre for motor vehicles, particularly for medium and heavy duty vehicles. This tread has a plurality of zigzag circumferential grooves delimited by a pair of shoulder ribs, located on opposite edges of the tread, and a plurality of circumferential rows of lugs delimited by transverse cuts having a width of less than 3 mm. The transverse cuts of each circumferential row of lugs are staggered circumferentially with respect to the transverse cuts of the adjacent row, and have an inclination in the range from 5° to 25° with respect to the direction perpendicular to the outer surface of the tread.
Furthermore, the inclination of the aforesaid cuts with respect to the normal direction of rotation of the tyre is such that, during travel, the reactions transmitted by the ground to the tread in the contact area tend to reduce this inclination with respect to said direction perpendicular to the outer surface of the tread, in order to provide a greater uniformity of wear of the lugs and to avoid the known phenomenon of “saw-tooth” wear U.S. Pat. No. 4,550,756 shows a pneumatic tyre having a ground tread portion, the tread portion comprising at least two circumferentially extending grooves, each groove having a pair of oppositely disposed sidewalls. The included angle between the second portion of the first groove sidewall and the second sidewall being at least 30°.
U.S. Pat. No. 3,858,633 illustrates a radial tyre for heavy-duty vehicles comprising a tread having a central portion and two lateral portions. The central portion is formed with at least one circumferential rib and at least two circumferential grooves, and each lateral portion is formed with a circumferential zigzag rib and with transverse grooves and blocks.